Aromatic polyamides (aramids) are known raw materials of high thermal and chemical stability and low flammability. For instance, fibers and sheets of such raw materials have very good mechanical properties, such as high strength and high initial modulus (modulus of elasticity), and are highly suitable for technical applications, for example for reinforcing plastics or for use as filter materials.
It is known that filaments or fibers can be produced from polyaramides with high strength and high initial modulus if the amide bonds on the aromatic nuclei are coaxial or almost parallel to each other, forming rigid, rodlike polymer molecules.
A typical polyamide of this kind is for example poly-(p-phenyleneterephthalamide). Filaments made of this material are described for example in German Patent 2,219,703.
This polyamide has a number of advantages, but its production and its processing are very difficult. Because this polymer is insoluble in polar organic solvents, even in the presence of inorganic salts, such as calcium chloride or lithium chloride, as solubilizers, this polymer, once formed, will quickly precipitate even from the reaction medium. It has to be isolated, washed, dried and then redissolved in a spinning solvent. The preferred solvent for preparing spinning solutions is concentrated sulfuric acid, which presents special problems with handling (safety, corrosion) and waste disposal.
Attempts have therefore been made to circumvent these difficulties by developing copolyamides which are readily soluble in the known amide solvents and which are also readily spinnable and whose filaments, after drawing, exhibit high strength values and initial moduli.
For instance, German Patent 2,556,883 and German Offenlegungsschrift 3,007,063 describe copolyamides of terephthalic acid, p-phenylenediamine and 3,4'-diaminodiphenyl ether, which, in amide solvents, form isotropic solutions of good spinnability. The filaments acquire high strengths and moduli on drawing to a very high draw ratio. The enhanced solubility is due here to the meta-orientation and the oxygen atom. Yet there continues to be a demand for aramids which are processible from known amide solvents into fibers having high strengths and moduli and which can be processed very economically into formed structures.
Also known are aramids comprising structural repeat units derived from 2-aminophenylamino-benzimidazole, -benzoxazole or -benzothiazole.
For instance, DE-B-2,530,875 (corresponding to U.S. Pat. No. 4,018,735) describes the production and processing of anisotropic forming solutions which contain such aromatic polyamides in high concentrations. There is a recommendation that the forming solution be produced using in particular concentrated sulfuric acid, but organic solvents are also mentioned. The aromatic polyamides described include copolyamides based on terephthalic acid, p-phenylenediamine and 5-amino-2-(p-aminophenyl)benzimidazole.
DE-B-2,211,241 discloses a process for producing high strength, heat-resistant fibers having a high modulus. It describes the spinning of aromatic polyamides containing heterocyclic units, including the spinning of a polyamide derived from 2,6-naphthalenedicarbonyl dichloride and 4,4'-diamine-2-phenylbenzoxazole.
DE-A-2,726,178 discloses a copolymer based on terephthalic acid, p-phenylenediamine and 5-amino-2-(p-aminophenyl)benzoxazole.
GB-A-1,341,945 discloses inter alia a polymer based on terephthalic acid and 6-amino-2-(p-aminophenyl)benzothiazole.
Furthermore, DE-C-2,208,811 describes aromatic polyamides derived from 2-aminophenylaminobenzimidazole and specific naphthalenedicarboxylic acids .